1. Field of the Invention
The present invention relates to magnetic sensors, using tunnel effect, for magnetic detecting devices or magnetic storage devices such as hard disk drives. The present invention particularly relates to a tunneling magnetic sensor which includes a free magnetic layer having low magnetostriction (λ) and which has a large change in reluctance (ΔR/R), high detection sensitivity, and high stability and also relates to a method for manufacturing the tunneling magnetic sensor.
2. Description of the Related Art
A tunneling magnetic sensor (tunneling magnetoresistive element) causes a change in reluctance using tunnel effect. When the magnetization of a pinned magnetic layer is antiparallel to that of a free magnetic layer, a tunnel current is prevented from flowing through an insulating barrier layer (tunnel barrier layer) disposed between the pinned and free magnetic layers, resulting in the maximum resistance. In contrast, when the magnetization of the pinned magnetic layer is parallel to that of the free magnetic layer, the tunnel current readily flows through the insulating barrier layer, resulting in the minimum resistance.
Since the magnetization of the free magnetic layer is varied by the influence of an external electric field, the tunneling magnetic sensor uses this principle to detect a change in electric resistance as a change in voltage, thereby detecting a magnetic field leaking from a recording medium.
Japanese Unexamined Patent Application Publication No. 11-161919 (hereinafter referred to as Patent Document 1) discloses a tunneling magnetic sensor including a pinned magnetic layer and free magnetic layer having a multilayer structure.
Japanese Unexamined Patent Application Publication No. 2005-191312 (hereinafter referred to as Patent Document 2) discloses a spin-valve magnetoresistive element including a free magnetic layer, a protective layer, and a spin filter layer disposed therebetween.
Japanese Unexamined Patent Application Publication No. 2006-5356 (hereinafter referred to as Patent Document 3) discloses a tunneling magnetic sensor including an aluminum oxide insulating barrier layer, a free magnetic layer, and a protective layer which includes an internal diffusion barrier sublayer, an oxygen-adsorbing layer, and an upper metal sublayer arranged above the protective layer in that order.
One of challenges for tunneling magnetic sensors is to enhance the sensitivity by increasing the change in reluctance to enhance properties of reproducing heads. One of techniques for increasing the change in reluctance of the tunneling magnetic sensors is that layers of materials having high spin polarizability are placed between free magnetic layers and insulating barrier layers.
Ferromagnetic materials, such as iron (Fe), nickel (Ni), and cobalt (Co), for forming the pinned and free magnetic layers of the tunneling magnetic sensors have slight distortion (magnetostriction) if the ferromagnetic materials are magnetized. An increase in the Fe content of alloys, such as a Ni—Fe alloy, a Co—Fe alloy, and a Ni—Co—Fe alloy, containing some of the ferromagnetic materials increases the spin polarizability and the change in reluctance. However, an increase in Fe content causes the free magnetic layers to have a large positive magnetostriction. An increase in the absolute value of the magnetostriction of the free magnetic layers causes noise in the reproducing heads, resulting in a problem that the reproducing heads have low stability. Hence, the absolute value of the magnetostriction is preferably small (nearly zero) and the change in reluctance is preferably large.
In the tunneling magnetic sensor disclosed in Patent Document 1, the free magnetic layer includes two Ni—Fe sublayers and a Co or Co—Fe layer. The composition of each Ni—Fe sublayer is appropriately adjusted and the Co or Co—Fe layer is placed between the free magnetic layer and an insulating barrier layer such that the free magnetic layer has low magnetostriction.
Patent Document 2 discloses that the change in reluctance of the magnetoresistive element can be enhanced by appropriately selecting a material for forming the free magnetic layer. In view of practical use, a Ni—Fe—Co alloy of which the composition is adjusted to reduce the magnetostriction is used to form the free magnetic layer.
As described above, the magnetostriction can be reduced by adjusting the material composition of the free magnetic layer or using such a low-magnetostriction alloy; however, the composition of the free magnetic layer that is adjusted to achieve low magnetostriction is not effective in achieving a large change in reluctance.
In the tunneling magnetic sensor disclosed in Patent Document 3, the internal diffusion barrier sublayer, oxygen-adsorbing layer, and upper metal sublayer of the protective layer are made of ruthenium (Ru), tantalum (Ta), and Ru, respectively. Therefore, this tunneling magnetic sensor has low magnetostriction and a large change in reluctance. However, the change in reluctance thereof is still insufficient.
The conventional structures described above are not effective in achieving low magnetostriction and a large change in reluctance.